This invention relates to overload protective systems and, in particular, to a novel overload protective system especially adapted for use on loads comprised in part of inductors having ferrous material cores.
Protective overload systems of the prior art, including those particularly suited to the protection of core type transformers, generally fall into either one, but not both, of the following categories: one category involves the insertion, in series, of a resistance between the load and the applied voltage on a temporary basis. Hence, when a voltage is applied across the serial connected load and resistance, the voltage across the load is smaller than it would be if the resistance were not in the circuit. Following such application, the resistance is effectively removed, as for example, by shorting, so that the full voltage is then applied across the load. The other category of system, which is likewise prevalent in the prior art, makes use of a zero crossover device such that a sinusoidal voltage is applied to the load at, or near, the time when the sinusoidal voltage is crossing the zero amplitude point. As is well known to those skilled in the art, a "zero crossover" device provides an indication when an A.C. signal passes thru zero, e.g., when it reverses polarity. By controlling the application time to correspond to a time when the amplitude is zero, or close to zero, the shock on the load is minimized.
As is well known, the permeability of a ferrous material of the type used to form transformer cores is not a constant. The permeability is usually represented by the symbol .mu.. The value of .mu. is equal to the ratio B/H; where B is the magnitude of the flux density, and where H is the magnitude of the magnetizing force. When the ferrous material is at a completely demagnetized state and current is applied to a winding in association with such material, the value of H increases as a function of the current and the value of B rises, first rapidly and then more slowly. A chart of the relationship between B and H would be termed an initial magnetization curve. At sufficiently high values of H, the curve tends to become flat. This condition is called magnetic saturation. When the field applied to such a material is increased to saturation and then is decreased, the flux density B decreases but not as rapidly as it increased along the initial magnetization curve. Thus, when H reaches zero, there is a residual flux density, or remanence, B.tau.. The phenomenon which causes B to lag behind H, so that the magnetization curve for increasing and decreasing fields is not the same, is called hysteresis, and a loop traced out by the magnetization curve is called a hysteresis loop. If the material is carried to saturation at both ends of the magnetization curve, the loop is called the saturation, or major, hysteresis loop. The residual flux density B.tau. of the saturation loop is called the retentivity, and the coercive force, H.sub.c, of this loop, is called the coercivity. Thus, the retentivity of a substance is; the maximum value which the residual flux density can attain, and the coercivity of a substance is the maximum value which the coercive force can attain. For a given specimen, no points can be reached on the B/H diagram outside of the saturation hysteresis loop, but, any point inside can.
When an inductor having a ferrous material core has been left in a condition where the residual flux density is at a maximum, such as at B.tau. (or at an opposite polarity condition such as at -B.tau.), then, when a voltage is applied across such a load in a direction which attempts to further saturate the core in the same direction, then the voltage applied thereacross would cause a relatively large current to flow through the inductor due to the fact that the inductor-core combination would be exhibiting a minimum impedance. In specific applications, an A.C. power source is connected to the power supply of a system through an off-on switch. When the A.C. power is applied to the power supply of the system, it is possible that the system's power transformer was left in a saturated condition, which condition, as previously stated, will cause an excessive amount of current to be drawn by the power transformer.